Ceramic heat exchanger structures



May 17, 1966 G. P. SMITH CERAMIC HEAT EXCHANGER STRUCTURES Filed Jan.

INVENTOR.

GA/L P. SM/TH Ace/ um. A77 0RNEK5 GM 1? 7 8mm,

United States Patent CERAMIC HEAT EXCHANGER STRUCTURES Gail P. Smith,Corning, N .Y., assignor to Corning Glass Works, Corning, N.Y., acorporation of New York Filed Jan. 5, 1962, Ser. No. 164,572 4 Claims.(Cl. 165-10) This invention relates to heat exchangers and in particularit concerns regenerative heat exchanger bodies particularly useful forapplication with gas turbines.

It is the primary object of the present invention to provide heatexchanger bodies composed of thin walled ceramic material, that arelightweight, are inexpensive, are characterized by a low coeflicient ofthermal expansion, and are chemically and thermally durable.

These and other objects are attained in accordance with this inventionin which a thin walled ceramic honeycomb is formed from ceramicmaterials hereinafter described in detail, and is provided with aprotective covering cemented to several of its surfaces. The honeycombbody is characterized by a plurality of unobstructed gas flow pathsextending through it to a pair of opposed, substantially parallelsurfaces. The term unobstructed as used in this application means thatthe flow paths have no internal structure that, for example, would stopgas flow. The unobstructed flow paths can be characterized as desiredand may be linear, tortuous or the like. These gas paths are separatedfrom one another by thin walls of the ceramic material. The specificdesign of the structure coupled with the character of the ceramicmaterials in the completed product result in a body uniquely suited as aregenerative heat exchanger for application at high and rapidly changingtemperatures such as are encountered in gas turbine operation.

The invention will be most readily understood upon considering itsdetailed description in conjunction with.

the attached drawing in which: a

FIG. 1 is a schematic representation of a heat exchanger and auxiliaryequipment showing the flow of gas therethrough;

FIG. 2 is a perspective view of a heat exchange body of the invention inwhich the channels are arranged for axial flow;

FIG. 3 is a perspective view of a second embodiment of the heat exchangebody in which the channels are arranged for radial flow; and

FIG. 4 is a perspective view of a honeycomb segment that can be used inmaking a radial flow heat exchanger.

Referring now to FIG. 1, fresh intake gas is fed by a line to acompressor 11 wherein its temperature and pressure are raised. The gasfrom compressor 11 then is passed through line 11a into a first portion12 of a.

heat exchanger body 13 heated to a high temperature as hereinafterdescribed. In that portion of the heat exchanger 13, the gas is heatedby the heat in the ceramic walls defining the gas flow paths and emergesat the opposite surface through line 14 at a higher temperature. Thehighly heated gas is then passed to :a burner 15 where its temperatureis further raised by combustion. The hot gas from the burner is thenexpanded through a turbine 16 where part of its energy is converted towork. The exhaust gas from the turbine is conducted through line 17 intoa relatively cool portion 18 of the heat exchanger body 13 where itgives up a substantial portion of its heat. Gas from this portion of theheat exchanger body is exhausted through a line 19. It is to be notedthat the heat exchanger body 13 is rotated in use; accordingly, afterone portion of it has been heated from the gases exhausted by theturbine, that heated portion rotates to the compressor inlet where thegas fed from the compressor can recover the heat. The two portions 12and 18 are divided from one another by the stationary seal 20 extendingacross the top and bottom surfaces by thin walls of ceramic material 26.At its hub or axial bore and along its outside surface, the honeycombbody is covered with, respectively, a hub wall 30 and a rim wall 32. Thehub and rim walls or members 30 and 32 are cemented to the honeycombbody by a cement 34 that is foamed in place. The heat exchanger body ofFIG. 2 can be .rotated by means, not shown, bearing on the hub, on therim or on both. I

For a radial flow heat exchanger, the gas channels I extend radially ofthe central axis of the cylindrical or annular shaped honeycomb bodysuch as is shown in FIG. 3. The honeycomb is characterized by aplurality of unobstructed gas flow paths 36 separated from one anotherby thin ceramic walls 37 and extending from the central wall 38outwardly and terminating in the outer surface 39. Where the honeycombof a radial flow heat exchanger body is essentially one piece, the topand bottom surfaces are covered with rim members 44 that are cemented tothe honeycomb structure by a foamed ceramic cement 46. Rotation of theradial flow heat exchanger body is achieved by use of members, notshown, that may apply rotational force on the rims.

For some purposes it is desirable to have the radial flow heat exchangerbody made of a plurality of segments. For example, if a part of the bodyshould be damaged, a segmented honeycomb body can be readily repaired bysimple replacement of the damaged segment. A segment of the type fromwhich such a honeycomb body can be assembled is shown in FIG. 4. In theembodiment of FIG. 4, the segment comprises a truncated pyramidal bodyhaving channels 50 extending between two of its parallel surfaces 51 and52. The other external surfaces are covered with a rim member 53 that,

as before, is cemented to the honeycomb body by a V achieved by usingcovering members, such as those shown at 44 in FIG. 3, cemented to theassembled segments.

Ceramic honeycomb bodies that are useful in accordance with theteachings of this invention can be prepared by several processes. Forexample, a pulverized ceramic material can be admixed with a suitablebinder and then extruded to a ribbon form. The resulting ribbon can befurther shaped if desired, and assembled, cit-her by itself or withother ribbons of this material to the desired honeycomb shape. Theresulting assembly is then sintered to a unitary structure. Preferably,however, the ceramic honeycomb body is prepared by coating a suitablecarrier with a mixture of a pulverized ceramic and a binder, crimpingthe resulting coated carrier and then assembling it to the desired shapealone or with another coated carrier that need not be crimped. Theassembled body is then heated to a temperature sufficient to sinter itto a unitary structure as more fully detailed hereinafter. This latterprocedure is, generally, the process set forth in the copendingapplication of Robert Z. Hollenbach, Serial 3 Number 759,706, filedSeptember 8, 1958, and now Patent Number 3,112,184 granted November 26,1963.

The purpose of the binder is to bond the unfired ceramic material to thecarrier, to impart green strength to the coated carrier and to retainthe formed unfired article in the desired shape after forming and priorto sintering. In order that the resultant article be essentially allceramic material having'a low coeflicient of thermal expansion, it ispreferred to use an organic binder, especially those that are heatcurable or thermosetting, that can be removed by decomposition and/ orvolatilization when the article is fired. Among the many materialshaving the requisite, well known characteristics of binders, that can beused in the process are such natural materials as gum arabic, colophonyand shellac and such synthetic organic resins as acrylate resins,methacrylate resins, alkyd resins, cellulose derivatives, coumaroneindene resin, phenolic resins, polyamides, polyesters, resorcinolresins, styrene resins, terpene resins, urea resins, vinyl resins,chlorinated paraffins and melamine resins.

The purpose of the carrier is to provide support for the unfired coatingto allow it to be formed to the desired shape prior to sintering theceramic coating. Tea bag paper is a preferred carrier and a list ofother suitable materials is disclosed in the aforementioned Hollenbachpatent application, to which reference can be made. Tea bag paper, aswell as other organic film materials, substantially decompose uponfiring and thus result in an article consisting almost entirely ofceramic material.

In order to produce a structure in accordance with the present inventionhaving characteristics suitable for a heat exchanger body, it isessential that ceramic materials be used that have a low coefiicient ofthermal expansion in the fired state on the order of about minus to plus10 times 10"'/ C. over an extended temperature range. Suitable ceramicmaterials for this purpose include lithium aluminosilicates such as, forexample, glass or crystalline petalite and beta spodumene,glass-ceramics having a lithium aluminosilicate base and especiallythose made in accordance with Example 1 of United States patent toStookey, Number 2,920,971, as well as mixtures of any of the foregoingmaterials. Petalite glass-ceramic mixtures generally include about 10 to40 weight percent of the glass-ceramic and the remainder petalite. Betaspodumene-petalite mixtures usually contain about 1 to 4 parts ofpetalite for each 4 to 1 parts of beta spodumene. These materialsnormally are used in a particle size of about minus 200 mesh (Tyler) orfiner, depending on the wall thickness desired in the resulting article.

Structures are assembled from ceramic coated carriers in a variety ofways, and the resulting structures are called honeycombs, a term whichin this specification means a unitary body having a multitude ofunobstructed gas paths of any predetermined size and shape, each suchgas path being defined by ceramic walls and extending to opposedessentially parallel surfaces. These structures can be assembled frommultiple layers of film corrugated with the same pattern with alternatelayers laterally disposed a distance equal to half of the width of theindividual pattern so that layers do not nest with each other. Thehoneycomb structure can also be formed from rolling alternate layers ofcrimped and uncrimped coated carriers until the desired shape is formed.A structure can also be formed by assembling to a stack alternatecrimped and uncrimped coated carriers until the desired dimensions areattained. Other .ways of assembling these honeycombs will be apparent tothose skilled in the art.

The firing of the green structure or matrix, however formed, isaccomplished in the normal manner for ceramic firing by placing thearticle in a furnace and heating it at a rate slow enough to preventbreakage due to thermal shock to a temperature high enough to cause theceramic particles to sinter. While the firing schedule, includingheating rates and sintering temperatures, will vary depending upon theceramic material utilized, the size and shape of the article formed, andthe atmosphere used, the details of such schedules are not critical andsuitable conditions are readily determinable by one skilled in the artof firing ceramic articles.

, As noted hereinbefore, the surfaces of the honeycomb body other thanthose where the gas flow paths terminate are covered. These coveringmembers, sometimes termed rims or hubs, can be formed in any desiredmanner. A preferred method of preparation involves forming a slip,suitably of the same composition as the ceramic used in the honeycombbody or of a glass such as the borosilicate glasses disclosed in theUnited States patent to Hood et a1. Number 2,106,744, and slip castingto the desired shape. Another suitable procedure involvesheat saggingstrips composed, for example, of the aforementioned borosilicate glass,to arcuate sections. Then several, e.g. four or more, arcuate sectionsare assembled to the desired shape and the abutting ends are joined byheat sealing.

The rim and hub or other cover members are attached to the honeycombbody by use of a ceramic cement that will foam and readily bond thosemembers and that has a low coefficient of thermal expansion in thefoamed state. Cement for this purpose has a composition, by weight, of 1to 16 percent of lead oxide, 1 to 15 percent of a flux, 1 to 6 percentof silicon carbide, 1 to 6 percent of S0 and substantially all of theremainder, and at least about percent of the total cement composition, alithium-aluminosilicate ceramic such as glassy or cystalline petalite, aglassy ceramic having such a base composition or any combination of theforegoing. Glass petalite is the preferred ceramic. Typical flux materials include the fluorides and oxides of magnesium, calcium,strontium, barium, zinc, cadmium, lead, lithium, sodium and pottassium.Suitably a mixture of oxide and fluoride fluxes is used. The S0 contentof the batch is provided by, for example, a compound such as calciumsulfate, barium sulfate, strontium sulfate or lithium sulfate. It willbe apparent that the use of any of these compounds provides both the S0and an oxide flux. Lead sulfate, which also may be used, provides theessential lead oxide and S0 The cement is used by pouring it into thespaces between the parts to be joined and then firing the unit to atemperature of about 1050 to 1150 C. until foaming and sintering arecomplete. Thereafter, the unit is cooled to handling temperature.

The invention will be described further in conjunction with thefollowing example in which the details are given by way of illustrationand not by way of limitation.

In this example, a ceramic composition is made of parts by Weight ofpetalite and 25 parts by weight of a glass-ceramic having the followingapproximate composition by oxide analysis in weight percent: 70 percentSiO 18 percent A1 0 5 percent TiO 3 percent Li O, 3 percent MgO and 1percent ZnO. The composition is ball-milled to a minus 200 mesh (Tyler)particle size. A solution of the following composition is added to 2160grams of the ceramic material in the ball mill:

640 cc. 860 cc.

Versamid is the trade name of a thermoplastic polymer supplied byGeneral Mills, Inc. It is prepared by condensation of polymerizedunsaturated fatty acids, such as dilinoleic acid, with aliphatic aminessuch as ethylene diamine.

The ceramic material and the binder are further ballmilled for aboutthree hours to produce a uniform suspension. A porous natural cellulosepaper, commonly known as 3 /2 pound tea bag paper, out to a width of 4inches is then dipped into the suspension and dried by heating to C. for2 minutes. The dried, coated paper is then heated to C. and crimped toproduce a pattern, taken in cross-section, in the shape of an isoscelestriangle with legs about 0.07 inch long and an open base about 0.1 inchwide. The crimped, unfired, coated paper is rolled up simultaneouslywith a sheet of tea bag paper of the same width, which has been coatedin the same manner but not crimped, upon a 2-inch diameter reel until anannular cylinder with an outside diameter of about 23 inches isobtained. Preferably, the uncrimped coated paper is not dried prior tothe roll-up operation, but this paper is dried by forcing air heated toabout 120 C. through the channels of the annular cylinder as they areformed during the roll up operation.

The unfired matrix body is then placed in a sealed furnace chamber andheated in accordance with the following schedule:

Temperature range: Firing rate Room temp. to 700 C. 350 C./hr. Hold at700 C n 1 hour. 700 C. to 1220 C. Furnace rate. Hold at 1220 C 30minutes. Cool to room temp. Furnace rate. Refire to 1240 C. 300 C./hr.Hold at 1240 C. 7 hours.

The sintered article is then cooled to handling temperature and removedfrom the furnace.

Rim and hub members are made for the foregoing honeycomb from a slip ofthe glass-ceramic used in preparing the honeycomb. The slip is cast inannular molds of a size to fit the honeycomb with about one-eighth inchclearance. After drying, the molds are removed and the cast structuresare fired at about 1250 C.

The rim, hub and honeycomb are assembled. Then a cement having thefollowing composition by weight is used to join these members: 9.23percent of zinc fluoride, 1.28 percent of calcium fluoride, 3.42 percentof silicon carbide, and the remainder petalite that had been fused withlead sulfate in an amount such that it contained 8 percent of lead oxideand 2.87 percent of S A batch of this cement composition is dispersed ina mixture containing 75 weight percent of butyl alcohol and 25 weightpercent of toluene and is wet ball-milled to thoroughly mix the batch.This cement is poured in the annular spaces in the assembly. Theresulting assembly is then placed in a furnace and raised to 100 C. at arate of 2 C./min. After 2 hours at 100 C., the temperature is raised at5 C/minute to 1t100 C. and is held at 1100 C. for one hour and 15minutes to permit the foaming action to be completed. It is then furnacecooled at a rate of 5 C./min. to handling temperature.

Honeycomb bodies formed to heat exchanger members as just detailed arecharacterized by an extremely large number of gas channels per unit ofsurface area. Quite commonly, a square foot of the surface of thehoneycomb will have over 57,000 channels. As is evident, therefore, mostof the cross section is void and the resulting units are relativelylightweight, having a density on the order of about 30 pounds per cubicfoot. These bodies made with the ceramic materials hereinbeforespecified have a low coefiicient of thermal expansion of about minus toabout plus 10 10* C. over a range up to about 300 C., and may actuallybe zero. It is accordingly, evident that these bodies can be subjectedto tremendous thermal shock as by repeated thermal cycling to over 1000C., a temperature beyond usual gas turbine operating temperatures,without adverse efiect. The excellent chemical durability of thestructure allows its extended use despite the corrosive conditionsexperienced in the application contemplated.

ized by a low coefficient of thermal expansion and therefore can readilybe produced to fixed dimensions and used, as by rotating it, withoutconcern about developing a poor fit. The invention is furtheradvantageous in that these unique results are achieved with readilyavailable and inexpensive raw materials while applying skills availableto artisans in the ceramic arts. Unless otherwise stated or apparent,all percentages and parts given in the foregoing description are byweight.

In accordance with the provisions of the patent statutes, I haveexplained the principles of my invention and have illustrated anddescribed what I now considered to be its best embodiment. However, Idesire to have it understood that, within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallyillustrated and described.

I claim:

1. A rotatable, annular-shaped heat exchanger body comprising a thinwalled ceramic honeycomb having two pairs of substantially parallelopposing surfaces and a plurality of unobstructed gas passages extendingbetween and terminating in a first pair of said opposing substantiallyparallel surfaces, said gas passages being defined by thin walls of saidceramic, rigid covering members on each of the other pair of opposingsurfaces, and a foamed ceramic cement joining said covering members tosaid honeycomb, said covering members, ceramic honeycomb and'foamedceramic cement all having a low and essentially similar coeflicient ofthermal expansion.

=2. A heat exchanger body in accordance with claim 1 in which saidunobstructed gas passages are essentially parallel to the central axisof saidheat exchanger body.

3. A heat exchanger body in accordance with claim 1 in which saidunobstructed gas passages are essentially radial to the central axis ofsaid heat exchanger body.

4. A rotatable, annular-shaped heat exchanger body formed of a pluralityof ceramic honeycomb segments each having a rectangular base pyramidalshape truncated parallel to its base and a plurality of unobstructed gaspassages extending between the base and its parallel surface, saidsegments being assembled to an annular shaped body with the collectedbases thereof constituting the external wall, a ceramic covering memberon each of the surfaces of each segment other than its base and thesurface parallel thereto, a foamed ceramic cement joining said ceramiccovering members to said honeycom'b segments, and means retaining saidassembled segments in position relative to one another, said ceramichoneycomb segments, said covering members and said foamed ceramic cementall having a low and essentially similar coefiicient of thermalexpansion.

References Cited by the Examiner UNITED STATES PATENTS 3,081,822 3/1963Wolansky et al. 10

FOREIGN PATENTS 750,303 6/ 1956 Great Britain. 811,434 4/1959 GreatBritain.

FREDERICK L. MATTESON, JR., Primary Examiner. CHARLES SUKALO, Examiner.

1. A ROTATABLE, ANNULAR-SHAPED HEAT EXCHANGER BODY COMPRISING A THINWALLED CERAMIC HONEYCOMB HAVING TWO PAIRS OF SUBSTANTIALLY PARALLELOPPOSING SURFACES AND A PLURALTIY OF UNOBSTRUCTED GAS PASSAGES EXTENDINGBETWEEN AND TERMINATING IN A FIRST PAIR OF SAID OPPOSING SUBSTANTIALLYPARALLEL SURFACES, SAID GAS PASSAGES BEING DEFINED BY THIN WALLS OF SAIDCERAMIC, RIGID COVERING MEMBERS ON EACH OF THE OTHER PAIR OF OPPOSINGSURFACES, AND A FOAMED CERAMIC CEMENT JOINING SAID COVERING MEMBERS TOSAID HONEYCOMB, SAID COVERING MEMBERS, CERAMIC HONEYCOMB AND FOAMEDCERAMIC CEMENT ALL HAVING A LOW AND ESSENTIALLY SIMILAR COEFFICIENT OFTHERMAL EXPANSION.